The present invention relates to a substantially wrinkle-free, non-planar thermoplastic film, and to a process for the manufacture of such film. More particularly, the present invention relates to a process for conforming a heat-shrinkable thermoplastic film having a high modulus of elasticity to the shape of a non-planar article without substantial formation of wrinkles in the conformed film. The process of the present invention is particularly suitable for the manufacture of automotive glazing and other transparent articles where optical clarity is important.
The construction and manufacture of automotive glazing products such as windshields, rear windows, sun roof panels, etc. is well known. One of the simpler glazing products may comprise a flexible interlayer of polyvinylbutyral sandwiched between a pane of glass on one side and a high modulus film, for example, polyester film, on the opposite side. This laminate is typically formed by assemblying the component layers and applying heat and/or pressure to permanently bond the layers together. European Patent No. 138,633 proposes an improved lamination process in which several "stacks" of components are simultaneously laminated.
Many automotive glazing products are not planar. Instead, the shapes of these products are becoming increasingly more complex in order to satisfy both aesthetic and aerodynamic demands of the automotive marketplace. While such curved automotive glazing products may satisfy certain aesthetic tastes and help minimize the overall wind resistance of the vehicle, they are extremely difficult to mass produce.
One source of difficulty relates to the production of high optical quality curved glass panels from commercial plate glass. Typically, such curved glass panels are manufactured by bending a pair of planar glass sheets as a pair so that their shapes conform to one another throughout the entire extent of the glass. A tunnel-like lehr is employed to heat the pair of glass sheets in unison to conform to the shaping surface of a bending mold. The production capacity of the lehr usually determines the overall production capacity of an automotive glazing manufacturing line. J. Golightly, "Producing Bent Laminated Windshields," U.S. Pat. No. 3,453,161 (July 1, 1969) discloses a method by which the production capacity of such lehrs may be increased wherein each bending mold is loaded with two pairs of glass sheets instead of one pair.
Wrinkling of the flexible interlayer sheet during its placement between the curved glass panels can also present a significant problem to the automotive glazing manufacturers. J. Valimont et al, "Vacuum Mold For Making A Windshield Subassembly," U.S. Pat. No. 4,501,546 (Feb. 26, 1985) discloses the use of a shaped, rotable vacuum mold, having curved walls conforming to the shape of the curved glass panels, to place a flexible interlayer themoplastic sheet such as PVB onto a curved glass panel in proper orientation and aligment without wrinkling. This patent does not address the wrinkling and optical distortion problems associated with the use of films possessing a high modulus of elasticity ("high modulus films"), such as biaxially-oriented polyester film.
High modulus films, such as biaxially oriented polyethylene terephthalate film, may form part of a protective layer for the automotive glazing laminate. Unfortunately, such films do not easily conform to many of the automotive glazing product shapes to which such films must be laminated. Instead, such high modulus films typically wrinkle, with the amount of wrinkling being directly dependent upon the complexity of the shape of the desired automotive glazing product. The wrinkling problem has prevented commercial acceptance of high modulus films as components of non-planar glazing products.
One reference which expressly deals with shaping a film in two directions of curvature is directed to metallized polyester film applications. D. Albert, "End Retract Device For Completing Spherically Shaped Reflective Film," U.S. Pat. No. 4,592,717 (June 3, 1986) discloses an apparatus in which the end portions of the film to be stretched are permanently attached to flexible stretch blocks such that the film wraps over a surface of the blocks which can be bent to arc positions matching the arc of curvature at the ends of a frame which carries the film in its final spherical configuration. The stretch blocks are then forced to flex about a neutral axis such that the surface of the blocks move about fixed end points from a chord configuration to an arc configuration which matches that of the ends of the stretching frame.